This invention relates to a novel method for reducing the degradation of an optical image in an exposure lighthouse. The method is particularly advantageous in preparing a viewing-screen structure for a CRT (cathode-ray tube), such as a color television picture tube, a display tube for an electronic information display, or the like.
Viewing-screen structures, such as black matrices or mosaic luminescent screens for use in cathode-ray tubes, have been made previously by photographic processes. A common process is carried out in an exposure lighthouse in which light from a light source, such as a high-pressure mercury vapor lamp, passes upwardly through one or more optical elements and a photographic master, which may be the apertured mask of the tube, and then is incident upon a photosensitive coating or layer on the inner surface of the viewing window of the tube, which window serves as a support for the layer. The light passing through the photographic master produces a light image, which exposes the photosensitive layer producing an image, corresponding to the light image, of exposed material that is either more or less soluble than the unexposed and underexposed areas, depending upon whether the material of the light-sensitive layer is positive acting or negative acting. In either case, the more-soluble portions of the exposed layer are removed during the subsequent development step.
It is desirable that the screen structure is a faithful replicate of the master, and efforts are made towards reducing as well as preventing the degradation of the optical image. Although the ambient atmosphere is conditioned to remove particles, the optical elements of the lighthouse must be cleaned frequently, as by flushing with compressed air or by brushing, to remove particles that deposit from the ambient atmosphere.
Also, it has been noted that liquid droplets also deposit on the optical elements. Some of the droplets appear to be the perspiration and/or saliva of the operators who load and unload the lighthouse. Such droplets dry on the surface of the optical element and then are difficult or impossible to remove by air flushing, brushing or even washing and/or wiping. The dry droplets degrade the light image by attenuating, refracting and/or diffracting local portions of the light image. The novel method reduces these degrading effects.